Roughly one half of the world's known coal deposits are located in North America and coal is the major fossil fuel resource of both North America and the world. There are two well known methods of mining coal. Coal deposits of great thickness at or near the surface are exploited by strip mining, and such deposits may account for five to ten percent of the known reserves. Strip mining generally causes serious environmental degradation, in that the top soil is removed and covered, the surface and under-surface drainage of the land is seriously disturbed, and strongly acidic compounds are commonly leached out of the material exposed after the overburden and coal are removed. Ecological restoration of the land is very expensive, and is rarely fully successful. The tendency is to regard the area mined as a sacrifice to economic necessity because of the large time lag, the high cost and doubtful result of land restoration after trip mining. Strip mining represents a destruction of the environment which is increasingly regarded as unacceptable. Secondly, where coal deposits are at a considerable depth, one thousand feet being typical, conventional deep mining techniques must be resorted to. The mining of coal deposits too deep to be stripped of overburden is costly and requires a large amount of manual labour. Coal mining is inevitably accompanied by a high incidence of accidents, caused largely by rock falls and gas explosions. In addition, the coal dust in the mine atmosphere causes severe lung problems, and it is well known that many coal miners are afflicted by black-lung disease. Furthermore, deep mining of coal is inefficient in that about only half of the coal is extracted, and that most of it is not mined at all, the seams being either too thin or too deep to permit economic working. There is also severe ecological degradation associated with deep mining. This is principally due to the amount of rock brought to the surface with the coal, and coal dust or other fumes.
In summary, present coal-exploitation methods are costly, dangerous, cause severe environmental damage, and extract only a small percentage of the total deposits. It is of greatest importance that coal be efficiently utilized but this is not possible with present methods of mining.